THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCT INTERNATIONAL APPLICATION PCT/JP99/05868 file date Oct. 25, 1999.
The present invention relates to a method for manufacturing rectangular cells, in particular to a method for manufacturing rectangular cells in which welding method for welding a sealing plate to an open end of a rectangular case containing a power generating element and hermetically sealing it is improved.
Rechargeable batteries to be used as a power supply for portable electronic devices are required to have a high energy density as well as a shape with good space-use efficiency to realize lighter weight and smaller size. As a type of batteries to satisfy these requirements, lithium-ion rechargeable batteries using aluminum case are drawing attention. As the lithium-ion rechargeable batteries are required to keep stable hermeticity over a long period because of their construction, an open end of a bottomed rectangular case is sealed by joining a sealing plate on which to form an electrode by laser welding. Compared with other welding methods, this laser welding method has an advantage of having superior working efficiency because of less thermal influence on the electrolyte contained inside the case or on electrically insulating parts.
Prior arts of manufacturing a rectangular cell by laser welding a rectangular case and a sealing plate as disclosed in Japanese Laid-Open Patent Applications No. Hei 8-315788, No. Hei 8-315789, and No. Hei 8-315790 are known. In the manufacturing methods disclosed in these applications, an open end of a rectangular case is hermetically sealed with a sealing plate by fitting the sealing plate into the open end of the rectangular case which is placed with its opening facing upward, and welding them by applying a laser beam in the vertical direction to the part where the rectangular case and the sealing plate are in contact and scanning the contact part with the laser beam.
However, as laser welding is a method for joining surfaces in contact with each other by fusing the portion irradiated with a laser beam, when there is a gap in the portion to be welded, welding defect such as a hole is easy to be caused on that place. Also, in the event a laser beam passing a gap is applied to a separator inside a cell which is weak against heat, though the output power may be weakened due to defocusing, these methods suffer a problem of causing a short circuit of the cell when the separator is burnt. There may also be a case where there is caused a displacement of the surfaces to be joined due to improper way of securing the surfaces to be joined. In such a case, depending on the angle of incidence of the laser beam, the applied laser output power is partially weakened as the beam is obstructed by displaced sealing plate thus resulting in insufficient fusion and making it difficult to secure required welding strength.
Furthermore, when using a rectangular case and a sealing plate formed with a thin sheet material for the purpose of achieving lighter weight and thinner design, and the part of contact of the sealing plate and the rectangular case is welded by applying a laser beam from the side of the upper open end of the rectangular case, there is a risk of molten metal entering into the cell when the amount of fusion has increased. As there is a limit in the precision of processing of the sheet material to be processed into a rectangular case and a sealing plate, even when the amount of fusion is set to an optimum quantity which will not reach inside the cell, a short-circuit may be caused when molten metal spatters to inside the cell in the event fusion reaching inside the cell takes place due to dispersion of precision of processing of thin sheet material.
The present invention has been contrived in view of the above described issues of the prior arts. It is an object of the present invention to enable joining of a rectangular case and a sealing plate by uniformly welding them without thermally affecting inside of the cell and furthermore to provide a method for manufacturing a rectangular cell with which the welding failure during mass production is reduced and productivity is dramatically improved.
In order to achieve the above object, the manufacturing method in accordance with the present invention is, in a method for manufacturing a rectangular cell in which a power generating element is contained in a bottomed rectangular cylindrical case with its upper open end formed into a shape having four straight sides and curved corners of a predetermined radius, and the upper open end of the rectangular case is sealed with a sealing plate by laser welding the sealing plate to the upper open end of the rectangular case, to make the sealing plate formed into the shape of the upper open end come in contact with the upper open end of the rectangular case, irradiate the line of contact of the rectangular case and the sealing plate with a laser beam from the side of the rectangular case in a slantingly downward direction, and weld the rectangular case and the sealing plate together by scanning the line of contact with the laser beam. In doing this, it is preferable that the outer dimension of at least either of the longer sides or the shorter sides of the sealing plate is made shorter than the outer dimension of the upper open end of the bottomed rectangular cylindrical case.
It is also preferable to form the sealing plate into the shape of the upper open end of the bottomed rectangular cylindrical case as well as to form a recessed portion which will fit into the upper open end of the rectangular case, make the rectangular case and the sealing plate come in contact with each other with the recessed portion fitted into the upper open end of the rectangular case, and apply a laser beam to the line of contact of the rectangular case and the periphery of the sealing plate from the side of the rectangular case in a slantingly downward direction. The angle of incidence of the laser beam is preferably in the range 5 to 45 degrees from the normal to the side of the case.
With this manufacturing method, as a laser beam is applied sideways of the cell case, the problem that the beam passing a gap in the surfaces to be joined directly impinges on and burns the separator can be solved. By making the outer dimension of the longer sides and the shorter sides of the sealing plate shorter than that of the upper open end of the rectangular case, and applying the laser beam from the side of the rectangular case in a slantingly downward direction, it is possible in mass production to precisely put together the surfaces of the sealing plate and the case to be joined as well as to remove any part that obstructs the laser beam, thereby improving the welding reliability.
Furthermore, by making the sealing plate come in contact with the upper end of the rectangular case by fitting the recessed portion formed on the sealing plate into the upper open end of the rectangular case, inside of the line of contact along which the rectangular case and the sealing plate come in contact with each other on the outer periphery is closed by the recessed portion thus preventing sputters from entering inside the cell when welding by scanning the line of contact with a laser beam from the side of the rectangular case.
Also, in the event the amount of fusion due to laser welding exceeds the sheet metal thickness of the rectangular case, entering inside the cell of molten metal due to fusion can be prevented as the amount of fusion in excess of the sheet material thickness reaches the recessed portion.